Added NewFromBigInt constructor & unit test (#56)

decimal.go

@@ -81,6 +81,14 @@ func New(value int64, exp int32) Decimal {
 	}
 }
 
+// NewFromBigInt returns a new Decimal from a big.Int, value * 10 ^ exp
+func NewFromBigInt(value *big.Int, exp int32) Decimal {
+	return Decimal{
+		value: big.NewInt(0).Set(value),
+		exp: exp,
+	}
+}
+
 // NewFromString returns a new Decimal from a string representation.
 //
 // Example:

decimal_test.go

@@ -231,6 +231,39 @@ func TestNewFromFloatWithExponent(t *testing.T) {
 	}
 }
 
+
+func TestNewFromBigIntWithExponent(t *testing.T) {
+	type Inp struct {
+		val *big.Int
+		exp   int32
+	}
+	tests := map[Inp]string{
+		Inp{big.NewInt(123412345),-3}: "123412.345",
+		Inp{big.NewInt(2234),	  -1}:      "223.4",
+		Inp{big.NewInt(323412345), 1}: "3234123450",
+		Inp{big.NewInt(423412345), 0}:  "423412345",
+		Inp{big.NewInt(52341235), -5}:  "523.41235",
+		Inp{big.NewInt(623412345),-6}: "623.412345",
+		Inp{big.NewInt(723412345),-7}: "72.3412345",
+	}
+
+	// add negatives
+	for p, s := range tests {
+		if p.val.Cmp(Zero.value) > 0 {
+			tests[Inp{p.val.Neg(p.val), p.exp}] = "-" + s
+		}
+	}
+
+	for input, s := range tests {
+		d := NewFromBigInt(input.val, input.exp)
+		if d.String() != s {
+			t.Errorf("expected %s, got %s (%s, %d)",
+				s, d.String(),
+				d.value.String(), d.exp)
+		}
+	}
+}
+
 func TestJSON(t *testing.T) {
 	for _, s := range testTable {
 		var doc struct {